Higher temperature solution processes for olefin polymerization are highly desirable due to the increased throughput, decreased energy necessary for devolatization and decreased fouling that these higher temperatures afford. Although Ziegler-Natta catalyst systems can be run at high temperatures commercially, these catalysts suffer from poor efficiency and poor comonomer incorporation at elevated temperatures. In addition, polymers produced from Ziegler-Natta catalysts at elevated temperatures have broadened molecular weight distributions, thereby limiting their suitability for use in many applications. Conventional Ziegler-Natta catalysts are typically composed of many types of catalytic species, each having different metal oxidation states and different coordination environments with ligands. Examples of such heterogeneous systems are known and include metal halides activated by an organometallic co-catalyst, such as titanium chloride supported on magnesium chloride, activated with trialkyl aluminum. Because these systems contain more than one catalytic species, they possess polymerization sites with different activities and varying abilities to incorporate comonomer into a polymer chain. The consequence of such multi-site chemistry is a product with poor control of the polymer chain architecture, leading to a heterogeneous composition. Moreover, differences in the individual catalyst site produce polymers of high molecular weight at some sites and low molecular weight at others, resulting in a polymer with a broad molecular weight distribution. Due to these reasons, mechanical and other properties of the polymers are often less than desired.
More recently, catalyst compositions based on well defined metal complexes, especially transition metal complexes such as constrained geometry catalysts (CGCs), metallocenes and post-metallocenes have been shown to give products having better comonomer incorporation and narrow molecular weight distribution. However, these catalysts often have poor high temperature stability and suffer from poor efficiencies at elevated polymerization temperatures. Additionally, the molecular weight of the polymers formed from these catalysts often decreases dramatically with increasing temperature, especially for polymers containing significant amounts of comonomer (lower density). That is, the ability of most olefin polymerization catalysts to incorporate higher α-olefins in an ethylene/α-olefin copolymer decreases with increasing polymerization temperature. In other words, the reactivity ratio r1 generally increases with increasing polymerization temperature.
Reactivity ratios of catalysts may be obtained by known methods, for example, the technique described in “Linear Method for Determining Monomer Reactivity Ratios in Copolymerization”, M. Fineman and S. D. Ross, J. Polymer Science, 5, 259 (1950) or “Copolymerization”, F. R. Mayo and C. Walling, Chem. Rev., 46, 191 (1950). One widely used copolymerization model is based on the following equations:
                                          M            1            *                    +                      M            1                          ⁢                  →                      K            11                          ⁢                  M          1          *                                    (        1        )                                                      M            1            *                    +                      M            2                          ⁢                  →                      K            12                          ⁢                  M          2          *                                    (        2        )                                                      M            2            *                    +                      M            1                          ⁢                  →                      K            21                          ⁢                  M          1          *                                    (        3        )                                                      M            2            *                    +                      M            2                          ⁢                  →                      K            22                          ⁢                  M          2          *                                    (        4        )            
where Mi refers to a monomer molecule which is arbitrarily designated as “i” where i=1, 2; and M2* refers to a growing polymer chain to which monomer i has most recently attached.
The kij values are the rate constants for the indicated reactions. For example, in ethylene/propylene copolymerization, k11 represents the rate at which an ethylene unit inserts into a growing polymer chain in which the previously inserted monomer unit was also ethylene. The reactivity ratios follow as: r1=k11/k12 and r2=k22/k21 wherein k11, k12, k22 and k21 are the rate constants for ethylene (1) or propylene (2) addition to a catalyst site where the last polymerized monomer is an ethylene (k1X) or propylene (k2X).
Thus, an olefin polymerization process is sought in which polymers containing various amounts of comonomer content can be produced with high catalyst efficiency and high monomer conversions and very high reactor temperatures without suffering from poor overall molecular weight in the resulting polymers. In addition, low molecular weight distribution (MW/MN<3.0) is desired in such a process. Ideally, such a process could be carried out at elevated temperatures and still produce polymers having high molecular weight and relatively high comonomer incorporation. It is known in the art that polymer molecular weight is readily controlled by use of chain transfer agents such as hydrogen or organometal compounds. Thus, a high temperature polymerization process that is capable of high levels of comonomer incorporation and produces high molecular weight polymers having low molecular weight distributions is desired in the art. Such a process additionally including a chain transfer agent to produce lower molecular weight polymers or the incorporation of long chain branching is further desired.
In US 2005/0215737 A1, a continuous, solution, olefin polymerization process is disclosed for preparing ethylene-butene and ethylene-propylene interpolymers at high ethylene conversions. Disadvantageously, the resulting polymers were primarily plastomers having relatively low molecular weights. No chain transfer agent was employed, indicating that molecular weight of the resulting polymer was relatively low and catalyst efficiencies were also low, especially at higher reaction temperatures.
In WO 99/45041, another continuous solution olefin polymerization process is disclosed using bridged hafnocene complexes with noncoordinating anionic cocatalysts. Although the resulting polymers contained significant amounts of comonomer, catalyst efficiencies were relatively low and polymer molecular weights, even in the absence of chain transfer agent were less than desirable.
In WO 03/102042, a high temperature solution olefin polymerization process is disclosed using indenoindolyl transition metal complexes to prepare polyolefins at temperatures at greater than about 130° C. In one example, the copolymerization of ethylene and 1-hexene was carried out at 180° C. resulting in formation of a polymer having poor comonomer incorporation (density=0.937 g/cm3) at relatively low catalyst efficiencies.
In U.S. Pat. No. 6,827,976, there are disclosed certain highly active polymerization catalysts comprising Group 3-6 or Lanthanide metal complexes, preferably Group 4 metal complexes, of bridged bi-aromatic ligands containing a divalent Lewis base chelating group. The metal complexes were employed in combination with activating cocatalysts in the polymerization of olefins including mixtures of ethylene and α-olefins, including 1-octene, to obtain polymers containing high comonomer incorporation rates at elevated temperatures.
US2004/0010103 disclosed certain aromatic polyether derivatives of transition metals and their use as catalysts for olefin polymerizations. Typical olefin polymerizations using prior art compositions are disclosed in US2003229188, WO00/24793, Akimoto, et al., J. Mol. Cat. A: Chem. 156(1-2), 133-141 (2000), among other references.
We have now discovered that certain metal complexes may be employed in a solution polymerization process to prepare relatively high molecular weight ethylene interpolymers containing relatively large quantities of comonomer incorporated therein at unusually high temperatures and high olefin conversions if certain process conditions are observed. Accordingly, there is now provided a process for the preparation of olefin polymer products, especially high molecular weight polyolefins, at very high catalyst efficiency. In addition, we have discovered that these catalyst compositions retain their high catalyst activity using relatively low molar ratios of conventional alumoxane cocatalysts. The use of reduced quantities of alumoxane cocatalysts (up to 90 percent or more less than conventionally employed) allows for the preparation of polymer products having reduced metal content and consequently increased clarity, improved dielectric and other physical properties. In addition, the use of reduced quantities of alumoxane cocatalysts results in reduction in polymer production costs.